In many installations for carrying out processes in which fluids are flowing through pipes, it is desired to maintain the piping and the fluid being carried at a temperature level above that of the ambient. Often this is accomplished by applying electrical resistance heaters to the outside of the pipes.
The prior art involves two basic types of resistance arrangements. In series resistance heaters, the resistance wire extends the length of the heater and then back again to the input terminals forming two series-resistance legs as shown in FIG. 1. It is to be understood that in both FIGS. 1 and 2 the resistance wires in the heater cable are insulated from each other and from the piping to which this heater cable is applied. This series resistance type heater has the advantage that it is easy to manufacture, but it suffers the disadvantage that a break at any point in either leg of the resistance circuit will interrupt the flow of current to the whole cable.
Another disadvantage with this series resistance heater is that there is no way to shorten the length of a given heater cable without increasing its thermal output for a given applied voltage. For example assume an applied voltage E of 120 volts, and assume that the heater cable is 100 feet long with a total resistance of 12 ohms. The current flow is then 10 amperes and the heat output calculated by I.sup.2 R is (10 amperes).sup.2 .times. (12 ohms) = 1,200 watts. Since the heater cable is 100 feet long, this means that the heat output is 12 watts per lineal foot of heater cable length.
If the heater cable is found to be too long for an installed pipe, the installation technician may wish to cut the cable length and rejoin the resistance wires at the cut end, as shown in FIG. 2. For example, if the cable is cut to 67 feet, the resistance becomes 8 ohms. The current flow for this same voltage E of 120 volts is now 15 amperes. The total heat output becomes (15).sup.2 .times. (8) = 1,800 watts. Since the heater cable is now 67 feet long, the heat output has become 27 watts per foot which is more than twice the heat output per foot even though the cable was only shortened by one-third. This effect means that a series resistance type heater cable cannot as a practical matter in most cases be cut to length in the field.
The second type of resistance heater cable is a parallel resistance arrangement as shown in FIG. 3 in which the terminals 10 and 12 are connected to a pair of conductors 14 and 16. The resistance wire 18 is arranged with multiple segments or legs 21, 22, 23, 24, 25, etc. extending in parallel electrical relationship between the conductors 14 and 16. In this parallel resistance type of pipe heater cable the heat output per lineal foot of cable is not affected by the length to which the cable is cut. For example, if the output of the heater cable as shown in FIG. 3 is 12 watts per foot for a given applied voltage E, the heat output per foot of active cable at applied voltage E remains the same if it is shortened by cutting it off at the dotted lines 28 or 29 or elsewhere.